1. Field of the Invention
The present invention relates in general to a tire-reinforcing steel cord and a pneumatic radial tire using the same, and more particularly relates to a tire-reinforcing steel cord and a pneumatic radial tire using the same which cord includes an inner layer consisting of 3 filaments and an outer layer consisting of 7 or 8 filaments and surrounding the inner layer, and in which the inner layer and the outer layer are twisted in such a manner that they have the same twist direction and the same twist pitch length, whereby the number of the twisting steps in the manufacture of the steel cord is reduced, and also the penetration of rubber into the steel cord is improved such that the steel cord has an improved durability upon application to tires.
2. Description of the Prior Art
Generally, an example of a steel cord used in tires for use with trucks or buses includes a two-layer steel cord which comprises an inner layer consisting of a plurality of twisted filaments, and an outer layer consisting of a plurality of twisted filaments disposed around the first layer while being integral with the inner layer. A further example of the steel cord used in tires for trucks or buses includes a three-layer steel cord which further comprises an outermost layer consisting of a plurality of twisted filaments disposed around the outer layer of the two-layer steel cord, and a spiral wrap consisting of a single twisted filament surrounding the outermost layer. The two-layer steel cord is generally used for a belt-reinforcing cord in tires, while the three-layer steel cord is generally used for a carcass-reinforcing cord in tires.
In brief explanation of a method for producing the two-layer steel cord, for example, 3 filaments are first twisted in either a right or a left direction to form an inner layer. Then, 7 or 8 filaments are twisted in a twist direction opposite to that of the inner layer and a twist pitch length different from that of the inner layer to form an outer layer, with the outer layer surrounding the inner layer while being integral with the inner layer. In the two-layer steel cord having such a 3+7 or 3+8 construction, the diameter of filaments of each of the inner and outer layers is generally in the range of 0.30 to 0.38 mm.
In such a two-layer steel cord, however, as the inner layer filaments and the outer layer filaments are different in a twist direction and a twist pitch length from each other, they are limited in contact area with each other. In other words, the filaments of the inner layer are in point contact with the filaments of the outer layer. Accordingly, when a tire, to which the two-layer steel cord is applied, is repeatedly flexed, a phenomenon may occur in which the mechanical friction between the filaments, and the chemical corrosion of the filaments due to the penetration of moisture or salt into the steel cord, are simultaneously generated. Such a phenomenon is called xe2x80x9cfretting fatiguexe2x80x9d. Such a fretting fatigue results in a degradation in the durability of the two-layer steel cord and the tire to which the steel cord is applied.
Moreover, in the two-layer steel cord, the inner layer and the outer layer are twisted in such a manner that they are different in a twisted direction and a twist pitch length from each other. Consequently, a twisting process is carried out for the inner layer and the outer layer, individually. This results in a reduction in productivity and an increase in the manufacturing costs.
Meanwhile, a representative standard for the three-layer steel cord is a xe2x80x9c3+9+15+Wxe2x80x9d construction. In brief explanation of a method for producing the three-layer steel cord having such a construction, 3 filaments are first twisted in a certain direction to form an inner layer. Subsequently, 9 filaments are twisted in the same twist direction as that of the inner layer while using a twist pitch length different from that of the inner layer to form an intermediate layer, with the intermediate layer surrounding the inner layer while being integral with the inner layer. After that, 15 filaments are twisted in a twist direction opposite to that of the intermediate layer while using a twist pitch length different from that of the intermediate layer to form an outer layer, with the outer layer surrounding the intermediate layer while being integral with the intermediate layer. Then, one filament is twisted in a direction opposite to that of the outer layer while using a desired twist pitch length to form a spiral wrap, with the spiral wrap surrounding the outer layer while being integral with the outermost layer. Generally, the filaments of the inner, intermediate and outer layers are equal in diameter to each other over the range of 0.175 to 0.250 mm, whereas the diameter of the filament of the spiral wrap is varied, if necessary.
As the three-layer steel cord is high in strength by virtue of a plurality of filaments and is great in binding force due to the presence of the spiral wrap, it is generally used for a carcass-reinforcing steel cord or a belt-reinforcing steel cord in large tires used for trucks or buses. Like the case of the two-layer steel cord, however, in the three-layer steel cord, the inner layer, intermediate layer, outer layer, spiral wrap are also twisted in such a manner that these layers have different twist pitch length and/or different twist direction between the adjacent layers. Consequently, in the manufacture of such a steel cord, a twisting process is carried out for the inner layer, intermediate layer, outer layer, and spiral wrap, individually. This results in an reduction in productivity and an increase in the manufacturing costs.
Therefore, the present invention has been made in view of the above mentioned problems, and, therefore, an object of the invention is to provide a tire-reinforcing steel cord and a pneumatic radial tire using such a steel cord which is improved in durability, thereby extending the durable life of the tire, and which is manufactured by a reduced number of processing steps for the twisting process, thereby reducing the manufacturing costs while achieving an improvement in productivity.
In accordance with the present invention, this object is accomplished by providing a two-layer steel cord including an inner layer consisting of 3 filaments and an outer layer consisting of 7 or 8 filaments and surrounding the inner layer, in which the inner layer and the outer layer are twisted in such a manner that they are equal in a twist direction and a twist pitch length to each other.
Moreover, the present invention provides a pneumatic radial tire including the steel cord in accordance with the invention in at least one of a carcass, a chafer and a belt thereof.